It is known to provide a master alloy (see U.S. Pat. No. 3,387,971) which has a molybdenum content of 20 to 25% by weight, a vanadium content of 20 to 25% by weight, no titanium, and the balance of aluminum.
This master alloy is formed in a single stage and its melting point is determined by the fact that the content of molybdenum plus vanadium plus aluminum is always at least 99% as a result of the limited content of carbon, oxygen, nitrogen, and hydrogen, to be less than 1400.degree. C. With a higher molybdenum content of the master alloy, however, problems arise in that molybdenum is only soluble with considerable difficulty in the titanium-based alloy.
The production of titanium-based alloys from master alloys deserves some comment.
Titanium-based alloys containing the elements aluminum, molybdenum and vanadium in different compositions and ratios are commercially significant because of their utility in the fabrication of aircraft and vehicles for space travel. Thus, it is especially important in the fabrication of titanium-based alloys that the alloying elements in the base metal be distributed with an optimum homogeneity so that properties of the metal bodies are substantially isotropic.
Especially metals having high melting points or refractory metals such as molybdenum with a melting point of 2610.degree. C. are difficult to dissolve homogeneously in the lower melting titanium whose melting point is only 1668.degree. C.
Experience has shown that existing aluminum master alloys containing molybdenum have not fully solved this problem. Such aluminum master alloys include Al.sub.12 Mo, Al.sub.5 Mo, Al.sub.3 Mo, Al.sub.2 Mo and AlMo.sub.3. Even with these alloys it is difficult to bring about complete and homogeneous dissolution of molybdenum, even in the form of the master alloy, in the titanium.
Undissolved molybdenum compounds and unmelted molybdenum particles, when distributed in the titanium-based structure, create problems in fabrication and as to the strength of the pieces made from the alloy because at the inclusion sites of the undissolved alloy or the particles, crack formation can occur. The aging properties of the product are poor, the fatigue resistance is low and, in general, practically all of the strength properties are adversely affected.
It is possible to approximate a satisfactory degree of homogeneity in titanium-based alloys by providing the alloying elements in appropriate master alloys and then mixing them with titanium sponge, and pressing the products at sufficient pressures to shaped articles. These shaped articles are then converted by welding in special processes to melting electrodes, which are transformed by electric arc furnace melting to ingots and, utilizing various ingot remelting techniques, the homogeneity of the resulting titanium-based alloys can be increased. These methods are extremely complex and frequently onerous.